Ophiolites and El Graara - Attribution of Data and Ideas.
School's out! Beautiful country, delightful people!
The most important early contributions to our knowledge of the El Graara and other inliers of the Anti-Atlas were those of Gilbert Choubert and Anne Faure-Muret. In particular their field guide to the Anti-Atlas, published in 1970, and their earlier (1962) map of the El Graara (Bou azzer) inlier, remain invaluable sources of information on the distribution of the Precambrian stratigraphic sequences and structural domains of the Anti-Atlas.
The El Graara ophiolitic rocks were described by Choubert (1963) as greenstones and migmatites. Choubert and Faure-Muret (1970, p 20) rejected the interpretation of the El Graara ultramafic-mafic rocks as ophiolites in the sense of Brunn and Routhier's 'epanchements sous-marins', whereas Routhier (1963), Bouladon et Destombes (1963) and Leblanc (1972) endorsed this view, considering the ophiolites to be large scale submarine mafic effusive units representing initial mafic magmatism during the development of an eugeosynclinal trough as per the model of Aubouin (1961; 1965). Leblanc (1972, p. 131) specifically rejected the model for ophiolite development as oceanic crust proposed by Reinhardt (1965) for the Oman ophiolite, and Leblanc's statement (Leblanc, 1972, p. 139) that the Bou Azzer ophiolite was situated 'sur la suture ophiolitique qui bordait de facon presque continue le craton ouest-africain' must be read in this context. The term 'suture' in this case marks the localization of the initial mafic magmatism of Aubouin’s eugeosynclinal model, not the suture of oceanic closure between colliding continents.
Elsewhere, the early speculations of Gass (1968) and Davies (1968) on the oceanic nature of the Troodos and Papuan ophiolites, respectively, and Reinhardt's (1969) recognition and attempt to explain the Oman sheeted diabase unit in terms of a mid-ocean ridge sea-floor spreading model, had led to the finding of sheeted diabase units in Troodos (Vine and Moores, 1969) and during the summer of 1969, based on a reading of Reinhard't paper, the sheeted diabase units of both the external and internal zone ophiolites of Newfoundland were recognized . In 1969 Dewey, and later Bird and Dewey (19770) published papers on the Caledonide-Appalachian system in which the ophiolites were interpreted as autochthonous arc-related intrusive rocks, whereas Church and Stevens (1970, Stevens 1970) interpreted the same rocks as transported slices of oceanic crust.
In 1972 Karig (1972) published his
classic paper on Remnant Arcs in the Pacific, formed as a result of the
collisional elimination of small back-arc basins formed by extension at the rear
of arc systems. From this point on there was a general tendency amongst
tectonics experts to interpret ophiolites as remnants of back-arc marginal
In attempting to link the geology of the Anti-Atlas to that of the Hoggar of Algeria, Caby and Leblanc (1973) suggested that the greenstones decorating the margins of the West-African Craton were of two types. The first included mafic complexes demonstrably injecting rocks of the West-African Craton, and represented a phase of crustal basification. Within the Pharusian geosynclinal trough of the Hoggar equivalent rocks include large amounts of quart diorite and lesser serpentinite. The second type is represented by an effusive series of basalts, spilites and keratophyres associated with the andesitic volcanogenic greywackes of the "Serie Verte". The andesitic source rocks overlie basified crust whereas the basalt/spilite/keratophyre are held to represent the upper units of an ophiolite similar to that found at Bou Azzer in the Anti-Adas. The ophiolites represent therefore the effusive component of the magmatism leading to crustal basification and oceanization of a zone extending from the Anti-Atlas to the Hoggar region. While this model obviously bears similarities to plate tectonic processes, it is nevertheless essentially autochthonist.
During an excursion to the El Graara inlier organised by Choubert in 1973 an apparent sheeted diabase unit was recognised at Ait Ahmane and the following year the finding were reported by Church and Young (1974) in support of the view that oceanic material was present in the Pan-African system of the Anti-Atlas. At that time Miyashiro (1973) put forward his controversial suggestion that the Troodos ophiolite represented an island arc rather than oceanic crust formed at a mid-ocean ridge, but this was largely rejected by most other Troodos workers. The problem was partly resolved by Rocci et al. (1975) who on the basis of their study of Alpine ophiolites, including Troodos, proposed a division of ophiolites into low-Ti (Troodos) and high-Ti types (true oceanic), whereas in the Appalachians, Church (1976) and Church and Riccio (1977) recognised a spectrum of types, including the Type I and type II ophiolites of Rocci et al., on the basis of the variation in the order of crystallization of the ultramafic-gabbro cumulates and of the relative variation of Ti with respect to FeO/MgO. Church and Coish (1976) pointed out on the basis of the Appalachian data that irrespective of the nature of the chemistry of the Troodos ophiolites, they nevertheless must have formed at a spreading centre.
In a very important and detailed study of the El Graara inlier, Leblanc (1975) accepted the evidence of the sheeted diabases as indicating that the Bou Azzer ophiolite had formed at an oceanic spreading ridge, but, quoting Miyashiro's (1973) conclusions concerning the arc origin of Troodos, contended that the quartz-diorites and keratophyres of the ophiolite belt were a penecontemporaneous and consanguinous part of the ophiolite. He also proposed (see also Leblanc and Billaud, 1978) that the ophiolite, although telescoped during Pan-African orogenesis, was located essentially in its original position of formation marginal to the Ebumean continent.
In 1978 the presence of boninites as basement beneath the trench-slope break of the Mariana arc was discovered by Meijer et al. and Dietrich et al.
A limited study of the dikes at the Oumarou section of the Bou Azzer ophiolite (Church 1980) showed that the dikes varied in character from low-Ti clinopyroxene-phyric to MORB to plagioclase-phyric calc-alkaline, whereas the low Ni relative to Cr abundance's in the altered gabbro screens may indicate that they were originally orthopyroxene bearing. Church also attempted to draw an analogy between the geology of the El Graara inlier and that of the western margin of the Appalachians, elaborating the view that ophiolite emplacement in Phanerozoic systems such as the Appalachians usually took place relatively early during ocean closure along a slip plane dipping oceanward, such that the present ophiolites must have been separated from the Ebumean continental margin by a significant stretch of oceanic crust. The arc component now recognised in such ophiolites would of course be related to the same phase of closure and the same slip plane.
The REE analysis of Bodinier et al. (1984) clearly showed that the effusive rocks could be divided into three main sets: a LREE depleted tholeiitic set which included the diabases of the ophiolite section at Oumarou and one sample from the volcanics north of the Tiddiline belt, a slightly LREE enriched set of two samples from within the main Graara belt (also sampled by Bloomer (Naidoo et al. 1991) in the Douissat region); and a moderately LREE enriched set of spilites from the Ousdrat and Skouraz regions with calc-alkaline arc affinities. One of the spilites from the Skouraz region showed a composition very similar to one of the plagioclase-phyric diabase dikes of the Oumarou section, claimed by Church to have calc-alkaline affinities. The microgabbro from the Oumarou ophiolite section is compositionaly similar to the diabase textured dikes analyzed by Church (1980) and has MORB characteristics, whereas the two diabase samples analyzed by Bodinier et al. were similar to the low-Ti clinopyroxene-phyric dike analyzed by Church. With respect to the presence of high-Ti basaltic rocks within the ophiolite belt it should be noted that the association of low Ti and high Ti basalts is a feature of the internal zone ophiolites of the Appalachians both in Quebec and Newfoundland. Bodinier also recorded the presence of only subordinate orthopyroxene in the layered olivine- plagioclase-clinopyroxene ultramafic-gabbro cumulates and plagioclase-clinopyroxene-orthopyroxene in the layered gabbros. The REE patterns of the gabbros all exhibited the positive Eu anomalies reflecting plagioclase accumulation, but whereas the less fractionated samples showed LREE depletion, the more fractionated gabbro sample showed a slightly concave upwards pattern. The keratophyres from the Ousdrat arc terrain also exhibit slightly concave patterns with marked negative Eu anomalies, feature reminiscent of boninites. Most important, the data indicated that the ophiolites and the quartz-diorites spatially associated with them were not consanguineous. Bodinier et al. concluded that the Bou Azzer ophiolite was formed in a marginal basin just behind a still active island arc. At the same time Moores et al. (1984) suggested an origin for the Troodos ophiolite also as a back-arc basin, whereas Pearce et al. (1984) introduced the term suprasubduction zone ophiolite for those ophiolites characterized by low-Ti abundances or boninitic characteristics, suggesting that they form above a subduction zone in the very early stages of arc development.
The presence in the Anti-Atlas of rocks belonging to the low-Ti ophiolite group was confirmed only following the description of ultra-low Ti mafic volcanic rocks associated with the Khzama ophiolite in the Siroua region by El Boukhari, A., Chabane, A., and Rocci, G., in a 1986 unpublished manuscript entitled 'Upper Proterozic Ophiolite in Siroua Massif (Anti-Atlas, Morocco): A Paleo-Transform Fracture Zone' (submitted to the Canadian Journal of Earth Sciences but regrettably never published). El Boukhari et al recognised the geochemical affinity of these rocks to the low-Ti rocks of the Troodos ophiolite, thereby confirming the tectonic similarity of the Siroua-El Graara ophiolites to the low Ti internal zone ophiolites of the Appalachians proposed by Church, but preferred to interpret their environment of formation as being an oceanic transform fault. At the same time Oshin and Crockett (1986) suggested that the very similar Thetford low-Ti boninitic ophiolite of the Appalachian system formed in a back-arc basin. In reviewing and recommending the El Boukhari et al. manuscript for publication, Church (1986) suggested to the authors that they consider the possibility that the Khazama and Bou Azzer formed in a spreading centre within the frontal part of a primitive oceanic arc adjacent to an obliquely subducting oceanic plate, rather than having formed in an oceanic transform fracture zone. This proposal was subsequently published (Church 1986) in a discussion of the origin of the Thetford ophiolite of the Appalachian internal zone. The existence of the El Boukhari manuscript and the interpretation of the Khzama rocks as boninitic was reported to Hefferan in a letter (June 7th 1990). Whether the petrographic and chemical characteristics of the low-Ti ophiolites can be reconciled in terms of an oblique subduction model remains to be evaluated.
Saquaque et al. proposed that the pre-PII-II rocks of the El Graara inlier are composed of a southerly accretionary prism (Skouraz, Menissi, Bleida), a northern fore-arc (Ousdrat), and a central uplifted sub-forearc basement (Bou Azzer, Oumarou ophiolite). The arc formed above a north dipping subduction zone and the quartz-diorites of the ophiolite belt were emplaced into the fore-arc as a result of ridge subduction.
In discussing the Saquaque et al. model for the development of the Bou Azzer ophiolite Church (1991) argued that the supposed Eburnean basement rocks of the Takroumt area, located north of the Siroua-Skouraz line of Church (1980), could represent "an eroded Eburnean continent or microcontinent, a pre-Pan African continent of some other age, or part of a Pan African oceanic or early continental margin arc. (The oceanic arc option has now been confirmed by D'Lemos et al. (2007).) He also argued that neither the arc sequences nor the quartz-diorites of the El Graara ophiolite belt need to have been formed above the same subduction zone that produced the ophiolitic rocks - nor need they have even been formed in a fore-arc environment. They could have formed as a result of south directed subduction prior to, or following obduction of the ophiolite - which must be true of the various quartz-diorite units within the arc complex and which have ages ranging from c. 650 to 750 Ma (Thomas, 2004, Inglis, 2005). Church further argued that the age and paleogeographic relationship of the ophiolites and Ousdrat - Skouraz -Menissi arc rocks was indeterminate: the calc-alkali arc rocks occur both south and north of the ophiolite belt and might either represent an arc formed directly on a basement of primitive SSZ ophiolite, or an arc that has been thrust over the latter. He also pointed out the uncertainty of correlating the Tiddiline and Trifya sedimentary sequences on opposite sides of Siroua-Skouraz line, given that the Trifya apparently contains debris from the adjacent c. 580 Ma Bleida intrusive rocks (see Leblanc, 1975, Fig. 44), whereas the relative age of the Tiddiline north of the line is uncertain. (For example Inglis et al. (2004) show the granitic rocks north of the 'Ambed' Tiddiline as equivalent to the 640 Ma Ousdrat quartz diorites whereas Bosquet et al (2008) show them as 580 Ma old post tectonic intrusions equivalent to the Bleida Granodiorite. ) Further, according to Choubert (1970, p. 116), the Tiddiline in the Ambed region (northern El Graara) is granitised by the northern quartz-diorite bodies, and is therefore at least older than 580 Ma.
Naidoo et al (1991) analyzed a comprehensive suite of sample from most of the pre-Tiddiline volcanic units of the El Graara inlier. The results largely replicated those of Leblanc (1975) and Bodinier et al. (1984), but emphasize the presence of an important component of high-Ti within-plate volcanic rocks in the Douissat area of the ophiolite belt, of evolved arc rocks in the southeastern Skouraz and west Menissi tracts, and of more primitive arc rocks in the western part of the Menissi tract. Of the diabase dikes analyzed by Naidoo et al. from the Oumaru ophiolite section, samples lklcF3Bc and K37cF2c closely resemble the high Zr, low Cr - Ni calc-alkaline samples 29-5 and 29-7, respectively, of Church (1980). These rocks were not represented in the Bodinier et al. data set. Sample lK20DYlb resembles the low-Ti sample 29-2, and samples DC8Dlc and 1-11 resemble the diabase textured dikes 29-3 and 29-6 of Church. Naidoo et al. also report the existence of two diabase dikes with elevated Ti02 values (2.27,3.24) perhaps representing fractionated MORB. Several of the calc-alkaline dikes cut the serpentinite as well as occurring in the diabase unit, thus confirming Church's surmise in this respect and indicating that the Ousdrat-Skouraz-Menissi arc rocks were laid down directly over the ophiolitic rocks.
The most recent model (Bousquet et al,. 2008) to speculate on the evolution of the El Graara rocks involves a continuous process of north to south obduction, between 750 and 660 Ma, of frontal ophiolitic arc and contemperaneous trailing calc-alkaline arc rocks over the Eburean continental margin, followed by north to south thrusting of the calc-alkaline component over the obducted ophiolite and coeval intrusion of a 650-640 Ma diorite suite. The 580 Ma granodiorite suite presumably represents a late-stage collisional adjustment. The model would appear to reject the possibility that any rock units of the El Graara inlier may have been formed by arc flip following the initial phase of arc obduction.
Historiographic bibliography (ordered chronologically):
Choubert, G. 1962. Carte Geologique de la Region de Bou Azzer - El Graara (Anti-Atlas Central) 1: 100000; carte No. 162 in Cat. Pub. de la Direction des Mines et de la Geologie.
Choubert, G. 1963. Histoire Geologique du Precambrien de l'Anti-Atlas: Tome 1, 352 p, 7 cartes couleurs.
Choubert, G., and Faure-Muret, A. 1970. Colloque International sur les correlations du Precambrien, Agadir-Rabt 1970. Guidebook: Anti-Atlas Occidental et Central, Notes et Memoires du Service geologique, no. 229, 259 p.
Church, W.R. and Young, G.M. 1974. L'excursion geologique de l'IGCP dans l'Anti
Atlas Marocain, Mai 1973. Geoscience Canada,
1, 1, p. 48-51.
Leblanc, M. 1975. Ophiolites precambrien et gites arsenies de cobalt (Bou Azzer, Maroc). Centre Geologique et Geophysique, Montpellier, Memoir Hors-Serie, 329 p.
Coment: LeBlanc's description of the Bou Azzer ophiolite was made in terms of
Routhier's pre-plate tectonic model of
'epanchements sous-marins' : Reinhardt's interpretation of the Oman ophiolite as allochthonous oceanic crust was explicitly rejected by Leblanc.
Jeanette, D. and Tisserant, D. 1977. Les episodes tectoniques et intrusif de Precambrien Superieur de l'Anti-Atlas occidental. Estudios Geologicas, 33, p. 315-326.
Leblanc, M. and Billaud, P. 1978. A volcano-sedimentary copper deposit on a continental margin of Upper Proterozoic age: Bleida (Anti-Atlas, Morocco). Economic Geology, 73, 1101-1111.
Ducrot, J., 1979. Datation a 615 Ma de la granodiorite de Bleıda et consequences sur la chronologie des phases tectoniques, metamorphiques et magmatiques panafricaines dans l’Anti-Atlas marocain. Bull. Soc. G´eol. Fr. 21, 495–499.***
Church, W.R. 1980. Late Proterozoic Ophiolites. Association mafiques-ultramafiques dans les orogenes. Colloques Internationaux du CNRS No. 272, p. 105-117.
Jeannette, D., Pique, A., 1981. Le Maroc hercynien: plateforme disloquee de craton ouest-africain. CR Acad. Sci. Paris 293, 79–82.
Leblanc, M., 1981a. The Late Proterozoic ophiolites of Bou Azzer(Morocco): evidence for Pan-African plate-tectonics. In: Kroner,A. (Ed.), Precambrian Plate Tectonics. Elsevier, Amsterdam, pp.435–451.***
Leblanc, M., 1981b. Ophiolites precambriennes et gites arseniesde Cobalt (Bou Azzer-Maroc). Notes et Memoires ServiceGeologique Maroc 280, 306.***
Leblanc, M. 1984. Metallogenesis of the Pan-African domain of the Anti-Atlas of
Morocco. Pan African Crustal Evolution in the Arabian Nubian Shield, I.G.C.P.
164, Faculty of Earth Sciences, King Abdulaziz University, Bulletin no. 6, p.
Bodinier, J.L., Dupuy, C. and Dostal, J. 1984. Geochemistry of Precambrian ophiolites from Bou Azzer, Morocco. Contrib. Mineral. Petrol., 87, p. 43-50.
(Liegeois, J.P., Bertrand, J.M., Black, R., 1987. The subduction- and collision-related Pan-African composite batholith of the Adrardes Iforas (Mali): a review. Geol. J., Winter Thematic Issue 22,185–211.)
Ighid, L., Saquaque, A., and Reuber, I. 1989. Plutons syncinematiques et la deformation panafricaine dans le Saghro oriental (boutoniere d'Imiter, Anti-Atlas). Paris, Academie des Sciences Comptes Rendus, 309, p. 615-620.
Saquaque, A. Admou, H., Karson, J., Hefferan, K., and Reuber, I. 1989. Precambrian accretionary tectonics in the Bou Azzer - El Graara region, Anti-Atlas, Morocco. Geology, 17, 12, p. 1107-1110.
Church, W.R. discuss. of Hefferan, K.P., Karson, J.A., Saquaque,A., Admou, H., and Reuber, I. 1991. Precambrian accretionary tectonics in the Bou-Azzer-El Graara region, Anti Atlas, Morocco. Geology, 19, 3, p. 285-286.
Extract: "Whereas Saquaque et al. believed the Bleida granodiorite to be syntectonic, Leblanc (1975, p. 143) stated that the 615 Ma Bleida body cuts Pan-African B1 folds and S1 schistosity, and that the contact minerals grew across the B1 microfolds and the S1 fabric." (The age of the Bleida granodiorite has now been revised to 579 Ma by Inglis et al (2004) and it is now conceded that the Bleida granodiorite was intruded during the later stages of Pan-African consolidation).
Hefferan, K.P., J.A. Karson, A. Saquaque, and I. Reuber, 1991.
Reply to comment
on Precambrian accretionary tectonics in the Bou Azzer-El Graara Region,
Anti-Atlas, Morocco by W.R. Church, Geology, 19, p. 286-287. "The Bleida
granodiorite, the youngest of the "granitoid" intrusions of the region, contains
a weak S1 schistosity that is generally parallel to the regional S1 schistosity.
Hence the Bleida granodiorite is interpreted to be synkinematic with the
sinistral shearing that produced the S1 schistosity (Saquaque et al., 1989b),
and does not postdate the the S1 schistosity as previously reported."
Naidoo, D.D. Blomer, S.H., Saquaque, A., and Hefferan, K. 1991. Geochemistry and significance of metavolcanic rocks from the Bou Azzer - El Graara ophiolite (Morocco). Precamb. Res., 53, 1/2, p.79-98.
Saquaque, A., 1991. Un exemple de suture d’arc: Le Precambrien del’Anti-Atlas
centre-oriental (Maroc). These Doctorat d’Etat, Universite Cadi-Ayyad,
Marrakech, p. 340.
Hefferan, K.P. Karson, J.A., and Saquaque, A. 1992. Proterozoic collisional basins in a Pan-African suture zone, Anti-Atlas Mountains, Morocco Precamb. Res., 54, 2/4, 295-.
Mrini, Z., 1993. Chronologie (Rb–Sr, U–Pb), tracage isotopique (Sr–Nd–Pb) des sources des roches magmatiques eburneennes, panafricaines et hercyniennes du Maroc. Unpublished Thesis. University of Marrakech, Maroc, 227 p.
Admou, H., 2000. Structuration de la paleosuture ophiolitique panafricaine de Bou Azzer - Siroua (Anti-Atlas central, Maroc).University Cadi Ayyad, Marrakech, 201 p.***
Hefferan, K.P., H. Admou, J.A. Karson and A. Saquaque, 2000. Anti-Atlas
(Morocco) role in Neoproterozoic Western Gondwana reconstruction. Precambrian
Res., 103, p. 89-96.
Reconstruction of latest Neoproterozoic Gondwana hinges on the interpretation of the subduction and collision kinematics of Pan-African orogenic belts that rim the West African craton. The Anti-Atlas suture zone of southern
Morocco has presented an enigma in this reconstruction as the inferred subduction zone polarity and age of suturing appear to be incongruous with better known West African orogens to the west (Mauretanian, Bassaride and
Rokelide) and Transaharan orogens to the east (Ougarta, Tuareg, Gourma and Dahomeyan). Contrary to previous interpretations, new data from the Anti-Atlas indicate a history of late Neoproterozoic ( 750–600) north-dipping
subduction culminating in the ( 600 Ma) collision of the Saghro magmatic arc to the north with the north-facing rifted margin of the West African craton. Thus, the Anti–Atlas suture links a 6000-km long chain of Pan-African
suture zones that essentially encircle the West African craton. The suture zones demarcate the former position of subduction zones that in all cases dipped away from the West African craton. The Anti-Atlas suture links the western
and eastern segments of the Pan-African orogenic belts associated with the amalgamation of Western Gondwana.
Charlot-Prat, F., Gasquet, D., Roger, J., Hassenforder, B., Chevremont,P., Baudin, T., Razin, P., Benlakhdim, A., Benssaou, M., Mortaji,A., 2001.Memoire explicatif, carte g´eol. Maroc (1/50,000), FeuilleSidi Bou’addi. Notes etMemoires Serv. Geol. Maroc, No. 415 bis.MICEM/BRGM.***
Ennih, N., Laduron, D., Greiling, R.O., Errami, E., de Wall, H.,Boutaleb, M., 2001. Superposition de la tectonique eburneene et panafricaine dans les granitoıds de la bordure nord du craton ouestafricain (Boutonniere Zenaga: Anti-Atlas central, Maroc). J. Afr. Earth Sci. 32, 677–693.***
Ennih, Nasser; Jean-Paul Liegeois 2001. The Moroccan Anti-Atlas: the West
African craton passive margin with limited Pan-African activity. Implications
for the northern limit of the craton
Precambrian Research 112, 289–302.
Department of Geology, Faculty of Sciences, BP 20, 24000 El Jadida, Morocco; Departement de Geologie, Section de Geologie, Musee Royal de l ’Afrique Centrale, Leuensesteenweg 13, B- 3080 Teruren, Belgium; http://www.metafro.be/Members/jpl/PR2001-AntiAtlas.pdf
Levresse G (2001) Contribution à l’établissement d’un modèle génétique des gisements d’Imiter (Ag–Hg), Bou Madine (Pb– Zn–Cu–Ag–Au) et Bou Azzer (Co–Ni–As–Ag–Au) dans l’Anti-Atlas marocain. Ph.D. thesis, CRPG-CNRS Nancy, France, p 191.
Hefferan, K., Admou, H., Hilal, R., Karson, J., Saquaque, A., Juteau T, Bohn M, Samson, S., and Kornprobst, J., 2002. Proterozoic blueschist-bearing mélange in the Anti-Atlas Mountains, Morocco: Precambrian Research, 118, 179-194
Blueschists from the Bou Azzer inlier provide compelling evidence for Late Proterozoic subduction in the Anti-Atlas Mountains of Morocco. High-pressure/low-temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar. Together with regional relationships, the geologic setting of the blueschists constrains the polarity of Pan African subduction in this region, which occurred from ~750 to 600 Ma. Blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix. The melange belt containing the blueschist facies rocks is intruded by a number of diorite plutons, one of which has yielded a U/Pb radiometric age of 650 Ma. Together with Transaharan Belt to the southeast, the Anti-Atlas suture zone exposed within the Bou Azzer inlier contains among the oldest known blueschist-bearing, ophiolitic melanges in the world.
Thomas, R.J., Gresse, P.G., Harmer, R.E., Eglington, B.M., Armstrong,R.A., de Beer, C.H., Chevallier, L.C., Martini, J.E., de Kock, G.S.,Fick, T., Macey, P.H., Ingram, B.A., 2002. Precambrian evolution of the Sirwa window, Anti-Atlas orogen, Morocco. Precambrian Res., 137, 1–57.***
Discussion by Lecolle et Derre of “The polygenetic Au–Ag bearing veins of Bou Madine (Jbel Ougnat, Eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit” [Abia et al., Journal of African Earth Sciences, 36 (2003) 251–271]
Reply to above Reply to discussion of “The polymetallic [not “polygenetic” …] Au–Ag bearing veins of Bou Madine (Jbel Ougnat, eastern Anti-Atlas, Morocco): Tectonic control and evolution of a Neoproterozoic epithermal deposit” [Abia et al., J. African Earth Sciences, 36 (2003) 251–271]
El Hassan Abia (a), Hassan Nachit (a), Christian Marignac (b), Abderrhamane Ibhi (a), Sai¨d Ait Saadi (a )
Liegeois, J.P., Latouche, L., Boughrara, M., Navez, J., Guiraud, M., 2003. The LATEA metacraton (Central Hoggar, Tuareg Shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny. J. Afr. Earth Sci. 37, 161–190.
Levresse G, Cheilletz A, Gasquet D, Reisberg L, Deloule E, Marty B, Kyser K (2004) Osmium, sulphur, and helium isotopic results from the giant Neoproterozoic epithermal Imiter silver deposit, Morocco: evidence for a mantle source. Chem Geol 207:59–79
Samson , S.D., Inglis, J.D., D'Lemos, R.S., Admou, H., Blichert-Toft, J. & Hefferan, K. (2004) Geochronological, geochemical, and Nd–Hf isotopic constraints on the origin of Neoproterozoic plagiogranites in the Tasriwine ophiolite, Anti-Atlas orogen, Morocco, Precambrian Research 135 , 133–147.
"Two of the plagiogranites were dated using high-precision U–Pb zircon techniques. Analyses of four batches of zircon from one pluton lie on a line with an upper intersection with Concordia of 761.1 + 1.9/−1.6 Ma, considered the age of emplacement of the pluton. Five small batches of zircon analyzed from a folded plagiogranite dyke lie on a chord that intersects Concordia at 762 + 1/−2 Ma, which is considered the age of emplacement of the dyke, and the most precise estimate of the age of the plagiogranite as a whole." (link at: http://earthsciences.syr.edu/Samson/Scott.htm (see comments re - El Boukhari above) Map (This map can be downloaded as a layer in the Google Earth .kmz file Morroco.kmz from: http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/ )
J Afr ES 2004
P, F. Oberli, J.-P. Burg, H. Nachit, J. Pons, M. Meier 2004. The
Palaeoproterozoic in western Anti-Atlas (Morocco): a clarification
Zircon U–Pb data on post-tectonic granites that have intruded the Precambrian formations of the Kerdous inlier (western Anti-Atlas, Morocco) are Pan-African. Zircon grains from a syntectonic monzodiorite of the Tahala pluton yield a precise discordia line with an upper intercept at 2043.9 ± 1.8 Ma (MSWD = 0.67) interpreted as the emplacement age. This age is consistent with zircon U–Pb ages of granites from other inliers and confirms the existence of a widespread magmatic event at c. 2045 Ma in the Anti-Atlas. The Pan-African event left no resolvable imprint on the isotopic systems of the analysed zircons. The Kerdous Schists are, therefore, older than 2045 Ma. Rb–Sr and Sm–Nd data on the schists and the Tahala pluton suggest that both juvenile and Archaean material contributed to the generation of the Palaeoproterozoic formations in the Anti-Atlas.
Bouougri, E.H., Saquaque, A., 2004. Lithostratigraphic framework and correlation of the Neoproterozoic northern Western African Craton passive margin sequence (Siroua-Zenaga-Bou Azzer El GraaraInliers, Central Anti-Atlas, Morocco): an integrated approach. J.Afr. Earth Sci. 39, 227–238.***
Gasquet, D., Chèvremont P, Baudin T, Chalot-Prat F, Guerrot C, Cocherie A, Roger J, Hassenforder B, Cheilletz A, 2004. Polycyclic magmatism in the Tagragra and Kerdous-Tafeltast inliers (western Anti-Atlas, Morocco). J Afr Earth Sci, 39 : 267–275.
Penaye, J.; Toteu, S.F.; Tchameni, R.; Van Schmus, W.R.; Tchakount, 2004, J.; Ganwa, A.; Minyem, D.; et. al. The 2.1Ga West Central African Belt in Cameroon: extension and evolution. pp. 159-164
Thomas, R.J., Fekkak A., Ennih N., Errami E., Loughlin E.S., Gresse P.G., Chevallier L.P., Liégois J.P. (2004) A new lithostratigraphic framework for the Anti-Atlas orogen, Morocco. J. Afr Earth Sci, 39 : 217–226
An example of the latter from near Bou Azzer has recently been dated at ~760 Ma ( Admou et al., 2002). In due course, the various components of the Bou Azzer Group may be given formal lithostratigraphic names, such as the Nqob and Khazama Ophiolites in the Sirwa area. All the ophiolitic fragments are fault-bound and were tectonically transported to their present positions during the later stages of the Pan-African Orogeny.
The third, allochthonous component of the early oceanic realm is composed of rocks that were produced in an island arc setting which formed by subduction of the oceanic crust north of the West African Craton margin ( Fig. 3). The products of the island arc comprise medium-grade biotite-rich (andesitic) schists (Tachoukacht Schists) and associated deeper level layered tonalitic orthogneisses (Iriri Migmatite). The formal term Iriri Group is proposed for this sequence. It also includes the Ourika Complex, a lithologically similar schist–gneiss assemblage exposed to the north, in the High Atlas of the Ourika valley. The Iriri Group rocks are always in tectonic contact with other units. The protolith of the Iriri Migmatite has been dated at 743 ± 14 Ma ( Thomas et al., 2002), confirming that it formed slightly after the ocean crust that is dated at ~760 Ma ( Admou et al., 2002; abst).
Inglis, J.D.; MacLean, J.S.; Samson, S.D.; D'Lemos, R.S.; Admou, H.; Hefferan, K. 2004. A precise U-Pb zircon age for the Bleida granodiorite, Anti-Atlas, Morocco: implications for the timing of deformation and terrane assembly in the eastern Anti-Atlas pp. 277-283
Precambrian rocks within the Bou Azzer inlier, southern Morocco expose the boundary between the West African Craton (WAC) and dismembered parts of the Anti-Atlas Pan-African orogenic belt. A pervasive greenschist fabric,is variably developed within Pan-African units, generally orientated 120/60 NNE. This fabric has been interpreted as recording thrusting of the Pan-African arc-complex onto the West African Craton. In the SE of the inlier an undeformed intrusion, the Bleida granodiorite, is emplaced within the allocthonous units of the arc-complex. The Bleida granodiorite crosscuts the predominant fabric elements in the host rocks and does not itself contain any pervasive fabric development. Because the host-rock fabric has regional tectonic significance, emplacement of the undeformed Bleida granodiorite necessarily occurred after cessation of thrusting and assembly of the tectonic blocks in the region. It hence constrains the cessation of the main period of Pan-African deformation in the eastern anti-Atlas. Two well constrained ages of 579.4 ± 1.2Ma and 578.5 ± 1.2Ma, based on U–Pb analyses of groups of 65 zircons, have been determined for two samples of the Bleida granodiorite. Penetrative regional deformation associated with collision of the arc-complex with the WAC was thus completed before ca. 580Ma. These new ages are closely similar to the ages of post-tectonic granites in the neighboring Sirwa inlier and therefore suggest that penetrative deformation in the eastern Anti-Atlas as a whole was near completion by ca. 580 Ma. See discussion and reply regarding the deformation state and relative age of the Bleida granodiorite.
Zetoutou, Souad; Ouzegane, Khadidja; Boubazine, Saliha; Kienast, Jean-Robert, 2004. Azrou NÃFad (Central Hoggar, Algeria) one of the deepest terranes of LATEA: arguments based on PT evolution in eclogite pp. 193-200
Gasquet, D., Levresse G, Cheilletz A, Azizi-Samir MR, Mouttaqi A (2005) Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times, with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition. Precamb. Res., 140, p.157–182
Essarraj, Samira; Marie-Christine Boiron , Michel Cathelineau , David A. Banks Mohamed Benharref 2005. Penetration of surface-evaporated brines into the Proterozoic basement and deposition of Co and Ag at Bou Azzer (Morocco): Evidence from fluid inclusions Jour Afr. Earth Sci, 41, p. 25-39 http://scholarsportal.info.proxy1.lib.uwo.ca:2048/cgi-bin/sciserv.pl?collection=journals&journal=08995362&issue=v41i1-2&article=25_posbitbaeffi&form=fulltext
Inglis, J.D.; R. S. D'Lemos, S. D. Samson, and H. Admou. 2005. Geochronological Constraints on Late Precambrian Intrusion, Metamorphism, and Tectonism in the Anti-Atlas Mountains. Jour Geology, 113, 439-450
The Bou Azzer inlier, Anti-Atlas Morocco, is a critical element for understanding Gondwanan geology because it exposes rocks formed during the paleosuturing of the Gondwanan margin with peri-Gondwanan arc terranes. Numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the Gondwanan margin to be bracketed. Detailed examination of plutons within the inlier reveal down temperature magmatic to solid-state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes. U-Pb geochronological data for the Aït Abdulla diorite (653.8±1.6 Ma) and Bou Offroh granodiorite (653.0±1.3 Ma) provide a limit to the onset of collision and regional metamorphism. A precise age of 640.8±1.4 Ma for the Ousdrat quartz diorite provides an indication of the lower limit of the collision in the region. Tectonothermal activity in the Anti-Atlas at ca. 650 Ma correlates with similar periods of tectonism in the peri-Gondwanan terranes of Avalonia and Cadomia and points to the emergence of continent-wide tectonism in western Gondwana as early as 650 Ma. ***
Mustapha El Ghorfi, Thomas Oberthür , Frank Melcher, Volker Lüders, Abdelmajid El Boukhari , Lhou Maacha, Rachid Ziadi and Hssain Baoutoul, 2006. Gold–palladium mineralization at Bleïda Far West, Bou Azzer–El Graara Inlier, Anti-Atlas, Morocco M ineralium Deposita Springer Berlin / Heidelberg ISSN 0026-4598 (Print) 1432-1866 (Online) Volume 41, Number 6 / September,
The lithostratigraphic–geodynamic framework and development of the Anti-Atlas of Morocco is a controversial topic (e.g., Choubert 1963; Leblanc and Lancelot 1980; Hefferan et al. 1992, 2000, 2002; Saquaque et al. 1989, 1992; Leblanc and Moussine-Pouchkine 1994; Thomas et al. 2004; Levresse et al. 2004; Gasquet et al. 2004, 2005), and work on modern, comprehensive models of crustal evolution and metallogenesis is still in progress. In general, the recently presented reconstruction of the Anti-Atlas by Thomas et al. (2004) and Gasquet et al. (2005), which regards the Anti-Atlas to represent a complex orogenic front that developed at the northern edge of the Eburnian (ca. 2.1–2.0 Ga) West African Craton during Pan-African times, can be followed. In the Anti-Atlas, the Precambrian basement comprises several Paleoproterozoic to Neoproterozoic units (traditionally subdivided into three “systems” (PI, PII, and PIII), which are unconformably overlain by late Ediacaran to Paleozoic rocks.
D’Lemos, R.S; J.D. Inglis , S.D. Samson, 2006., A newly discovered orogenic event in Morocco: Neoproterozic ages for supposed Eburnean basement of the Bou Azzer inlier, Anti-Atlas Mountains. Precambrian Research 147 (2006) 65–78.
" Southern parts of the Bou Azzer inlier include meta-igneous units, paragneiss, schist and muscovite-bearing granite that have until now been regarded as the c. 2 Ga Eburnean WAC basement onto which the ophiolite was obducted. An example of supposed Paleoproterozoic basement occurs at Tazigzaout. The main units within this complex are metagabbro, augen granite gneiss, finely banded granite gneiss and variably sheared syn-tectonic leucogranite, all of which are intercalated within a complex sub-vertical, upper greenschist facies, ductile dextral shear zone. The meta-igneous rocks are crosscut by a sheeted muscovite-biotite granite pluton. Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of 753 +1/−2 Ma. Zircon from a nearby metagabbro provide an indistinguishable date of 752.2 +/-2.4 Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two crosscutting leucogranite bodies provide younger dates of 705 +2/−3Ma and 701 +2/−1 Ma.
The initial εNd values of all lithological units in the complex range from +4.9 to +6.0, with depleted mantle model (TDM) ages of 890–790 Ma. The high, positive εNd values for all of these igneous units is indicative of an isotopically juvenile source, such as depleted mantle derived material, and precludes substantial interaction with old, evolved crustal sources. These data demonstrate that the “basement” rocks of the Tazigzaout complex are Neoproterozoic intrusions, not c. 2 Ga Eburnean units, as found in more southerly Anti-Atlas inliers. The structural relationships and U–Pb ages are interpreted as recording dextrally transpressive deformation of c.755 Ma juvenile crust after c. 750 and before 700 Ma. This Neoproterozoic deformation occurred as much as 100 Ma prior to the top-to-the-north thrusting of the meta-igneous units over units of the ophiolite and arc, and provides strong evidence for two discrete ‘Pan African’ events in Morocco."
Bousquet R., El Mamoun, R., Saddiqi, O. & Goffé, B., 2008. Mélanges and ophiolites: was the Bou-Azzer’s ophiolite suite (Morocco) a Franciscan-type wedge during the Pan-African orogeny? in The Boundaries of the West African Craton Eds Ennih, N. & Liégois, J.-P., Geological Society, London, Special Publications, 297: 233-247. http://www.geo.uni-potsdam.de/mitarbeiter/Bousquet/papers/bouazzer/ Map: http://instruct.uwo.ca/earth-sci/fieldlog/pan_african/maroc/Bousquet_08_7.jpg Interpretive section: http://instruct.uwo.ca/earth-sci/fieldlog/pan_african/maroc/Bousquet_08_9.jpg
From: Mustapha El Ghorfi, Thomas Oberthür , Frank Melcher, Volker Lüders, Abdelmajid El Boukhari , Lhou Maacha, Rachid Ziadi and Hssain Baoutoul, 2006. Gold–palladium mineralization at Bleïda Far West, Bou Azzer–El Graara Inlier, Anti-Atlas, Morocco Mineralium Deposita Springer Berlin / Heidelberg ISSN 0026-4598 (Print) (Online) Volume 41, Number 6 / September, p. 1432-1866
The above map can be downloaded as a layer in the Google Earth .kmz file Morroco.kmz from: http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/
Google Earth image of the Bleida region of the El Graara inlier showing localities of geological interest. The white outcrops in the south-centre of the image are the 580 Ma old Bleida granodiorite (compare with image above), and the green rocks to the south of the granodiorite are the north dipping Trifya mollassic sediments containing, according to Choubert, clasts of the granodiorite. The Trifya, which is commonly correlated with the Tiddiline is therefore younger than 580 Ma. The Ait Ahmane gabbro - sheeted diabase unit is located at the very top left of the image. The iron-rich 'oligistoschiste' unit is located at the base? of the Calcareous Schist unit (centre left).http://instruct.uwo.ca/earth-sci/fieldlog/pan_african/maroc/http://instruct.uwo.ca/earth-sci/fieldlog/pan_african/maroc/
Recent high resolution zircon age dates:
Kerdous Tahala Pluton
Kerdous Tahala PlutonBarbey, 2004 2044 Ma
granite gneiss D'Lemos,
2007 753 +1/-2 Ma - Tazigzaout Metagabbro
2007 752.2 +/- 2.4 Ma Tazigzaout leucogranite
Tamazzarra Granite GK 230 2037+/-9 c. 2170 (i)
Tasriwine ophiolite Samson, 2004 762
Tazigzaout augen granite gneiss D'Lemos, 2007 753 +1/-2 Ma -
Tazigzaout Metagabbro D'Lemos, 2007 752.2 +/- 2.4 Ma
Tazigzaout leucograniteD'Lemos, 2007 705 +2/-3 and 701 +2/-1
Island-arc related rocks
Thomas, R.J. et al., 2002 Ait Abdullah diorite
Ait Abdullah dioriteInglis, 2005 653.8
Bou Offroh granodiorite Inglis, 2005 653
Ousdrat diorite 2005 Inglis, 20005 640.8
Rhyolites associated with the Bou Salda Formation
Thomas, R.J. et al., 2002
Post Sarhro Group intrusive rocks
Thomas, R.J. et al., 2002 Bleida granodiorite
Bleida granodioriteInglis, 2004 579.4
Bleida granodiorite Inglis, 2004 578.5
Ouarzazate Group rhyolites Thomas, R.J. et al., 2002
Syn-Ouarzazate Group granites
Thomas, R.J. et al., 2002
2045 +/-10 (i)